This invention relates to an actuator operated by oil or compressed air and more particularly to a cylindrical actuator with a mechanism for detecting displacement of a piston moving in the actuator.
In general, servo-motors or hydraulic or pneumatic cylinders have often been used for actuating the arms of industrial robots. The servo-motors are used, in many cases, for controlling accurately the arms thereof because a feed back control can be easily carried out.
On the contrary, in the case of the hydraulic or pneumatic cylinders, as displacement of pistons moving in the cylinders cannot be detected accurately, the cylinders are mainly used for robots requiring a simple movement.
However, it is generally understood that the hydraulic or pneumatic cylinders are more suitable for actuators generating a relatively large amount of force than the servo-motors.
The Japanese Utility Model Laid Open Publication 16489/1980 discloses a cylindrical actuator with a mechanism for detecting displacement of its piston. In the Publication, the piston detecting displacement mechanism has a cylindrical detecting body which may form an induction coil and a core moving in the body. The detecting body and the core are accommodated in a hole formed through the center portion of a piston rod with a piston which is adapted to move in an outer casing. The core is fixed to the piston rod while the detecting body is fixed to the casing. In this construction, when the core is moved together with the piston, the relative positional relationship between the core and the detecting body is changed. The displacement of the piston, at that time, is detected on the basis of the change of the coil inductance.
In this arrangement, however, because the detecting body and the core are accommodated in the piston rod, the overall volume of the conventional actuator unavoidably becomes bulky.